Exploiting Vertical Diversity in Vehicular Channel Environments Sangho Oh, Sanjit Kaul, Marco Gruteser Electrical & Computer Engineering, Rutgers University, 94 Brett Rd, Piscataway NJ 08854 Email: {sangho, sanjit, gruteser}@winlab.rutgers.edu Abstract—Antenna diversity is a well-known technique used to improve the quality and reliability of a wireless link. In vehicular networks, a different approach to antenna diversity is needed due to their unique channel characteristics. However, this issue has not been actively researched, especially for the positioning of antennas. In this paper, we highlight the benefit of vertical diversity over traditional horizontal diversity techniques in vehicular network environments. Through experiments using IEEE 802.11a radios in the 5.2GHz band, we first show the difference of attenuation patterns from various antenna positions installed in a vehicle, then we show the benefit of vertical diversity by quantifying the diversity gains and combined error rates. This finding has implications for the future position of antenna installation in vehicles. Index Terms—antenna diversity, vertical diversity, vehicular network channel, ad-hoc network, broadcasting, two-ray ground model, height diversity I. I NTRODUCTION Vehicular Ad-Hoc Network (VANET) has been studied over a decade as an important component in Intelligent Transporta- tion Systems (ITS) that aim to provide comforts and safety to drivers and passengers on the road. Motivated by this, many projects have been launched related to the communications in VANET environments, e.g., Wireless Access in Vehicular Environment (WAVE) [1], PEeVENT [2], FleetNet [3], and Network on Wheels (NoW) [4]. They have actively developed various protocols that are specifically designed for vehicular network conditions. Among many research topics on VANET, reliable com- munications in Vehicle-to-Vehicle (V2V) and Vehicle-to- Infrastructure (V2I) is one of the fundamental issues, which is very closely related to the wireless channel characteristics. Although mobile communication channels have been thor- oughly investigated over several decades in cellular network systems, VANET channel characteristics have only recently been studied from the perspective of mobile Ad-hoc networks. VANET has unique channel characteristics. VANET com- munications experience location dependent outage probabili- ties due to a strong interference from the ground reflections [5] even when a link has a Line-Of-Sight (LOS) connection. Interference from a strong out-of-phase ground reflected signal generates a number of deep fades at specific locations — outage spots — between the transmitter and the receiver. The receiving vehicles at the outage spots will experience temporal disconnections. For road safety applications, such as collision warning or pothole alarms, the outage spots can lead to critical situations. In such cases, antenna diversity techniques exploiting spatial diversity can be used to improve the reliability in communica- tions. However, although the position of antenna for diversity greatly affects the performance of communication devices in VANET, this issues has not been thoroughly investigated considering the VANET specific channel characteristic. In this paper, we show using experimentally collected data from a US highway that vertical antenna diversity better copes with strong interference from ground reflections than horizontal antenna diversity. We first show the channel char- acteristics caused by a strong ground reflections through a controlled parking lot experiment, then move on to the road experiments on a highway to verity how vertical diversify improves the reliability of communications. By quantifying the gains from vertical diversity over horizontal diversity for various transmitter-receiver distances, we demonstrate the benefits of vertical diversity. II. RELATED WORK Andrisano builds a propagation model for V2V channels considering the interference from ground reflections and multi- path fading in the spectrum of millimeter wave (60GHz). He finds the outage probability [6], [7] between two vehicles on a road. Ebine and Yamada measured the gains from a vertically spaced diversity antenna mounted on a vehicle [8]. However the system is a cellular system utilizing tall base station antennas, and the spatial diversity gain is achieved from the diversity in radiation patterns. Verdone proved the suitability of a multi-hop network approach for Road Transport Information (RTI) applications in 60GHz band, which is based on R-ALOHA protocol [5]. He assumed two-ray Rician fading channels with vertical diversity in V2V links. He showed how vertical diversity helps the connectivity in multi-hop protocols in VANET through simulations, but has not conducted an experimental validation. Kaul et al., conducted experiments on the diversity gains in vehicular network environments by comparing the azimuthal patterns of each of antennas that are horizontally and vertically displaced on top of the roof of a vehicle [9]. They discuss traditional diversity gains from horizontally displaced antennas over small scale fading in VANET environments. 978-1-4244-5213-4/09/ $26.00 ©2009 IEEE 958